Optical system having a diffractive optical element, and optical apparatus

ABSTRACT

An optical system includes a diffractive optical element having a diffraction grating provided, on a lens surface having curvature, in a concentric-circles shape rotationally-symmetrical with respect to an optical axis. A sign of the curvature of the lens surface having the diffraction grating provided thereon is the same as a sign of a focal length, in a design wavelength, of a system composed of, in the optical system, a surface disposed nearest to an object side to a surface disposed immediately before the lens surface having the diffraction grating provided thereon, and is different from a sign of a distance from the optical axis to a position where a center ray of an off-axial light flux enters the lens surface having the diffraction grating provided thereon. Further, an apex of an imaginary cone formed by extending-a non-effective surface of the diffraction grating is located adjacent to the center of curvature of the lens surface having the diffraction grating provided thereon.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical system having adiffractive optical element, and more particularly to an optical systemsuited to optical apparatuses, such as film cameras, video cameras,digital cameras, telescopes, projectors, etc., in which a diffractiveoptical element and a refracting optical element are combined to effectachromatism well.

[0003] 2. Description of Related Art

[0004] Heretofore, as one of methods for correcting chromatic aberrationof an optical system, there is a method of combining two glass materials(lenses) which differ in dispersion from each other.

[0005] As against such a conventional method of combining the two-glassmaterials to diminish chromatic aberration, there is a method fordiminishing chromatic aberration by providing, at a lens surface or apart of an optical system, a diffractive optical element, such asa-diffraction grating, having a diffracting function, as disclosed inSPIE Vol. 1354 International Lens Design Conference (1990), JapaneseLaid-Open Patent Application No. Hei 4-213421 (corresponding to U.S.Pat. No. 5,044,706), Japanese Laid-Open Patent Application No. Hei6-324262 (corresponding to U.S. Pat. No. 5,790,321), U.S. Pat. No.5,044,706, etc.

[0006] This method is based on the utilization of such a physicalphenomenon that a refractive surface and a diffractive surface in anoptical system cause the behavior of chromatic aberration with respectto a ray of light of a certain reference wavelength to occur inrespective opposite directions. Further, it is possible to make such adiffractive optical element have an aspheric-lens-like effect by varyingthe period of the periodic structure thereof, thereby greatlyeffectively lowering aberration.

[0007] Here, while, in the case of refraction, one ray of light remainsbeing one ray of light even after being refracted, one ray-of light, inthe case of diffraction, is divided into a number of rays of variousorders after being diffracted. Therefore, in a case where a diffractiveoptical element is used in an optical system, it is necessary that thegrating structure of the diffractive optical element is decided in sucha manner that light fluxes included in a useful wavelength regionconcentrate on one particular order (hereinafter referred to also as adesign order), and it-is necessary that the diffractive optical elementhas the diffraction efficiency excellent over the entire image plane.

[0008] With regard to the diffraction grating, there is proposed, inJapanese Laid-Open Patent Application No. Hei 10-268115 (correspondingto U.S. Pat. No. 5,995,286), an optical system using a diffractiongrating of the blazed shape to aim at the evenness of the diffractionefficiency over the entire observation image plane.

[0009] In the above Japanese-Laid-Open Patent Application No. Hei10-268115, there are disclosed a Keplerian viewfinder optical systemarranged such that a diffraction grating of the blazed shape in whichthe height of a grating part at a marginal area of the diffractiongrating is less than the depth of a grating part at a central area,around an optical axis, of the diffraction grating is used to make thediffraction efficiency at the central area approximately equal to thatat the marginal area, and a Keplerian viewfinder optical system arrangedsuch that a diffraction grating of the blazed shape in which anon-effective surface (a surface having no diffracting function in thediffraction At grating and corresponding to a side surface of thediffraction grating) in a central area around an optical axis is formedas a part of a cylindrical surface and a non-effective surface of amarginal area is formed as a part of a conical surface is used toprevent the shading of a ray of light at the non-effective surface inthe marginal area.

[0010] In the Keplerian viewfinder optical systems as proposed in theabove Japanese Laid-Open Patent Application No. Hei 10-268115, anon-axial light flux and a most- off-axial light flux are separate fromeach other when passing through the position of a diffractive opticalsurface provided at a position relatively distant from a stop.Accordingly, this construction has such a characteristic that, withoutthe above state of passing-through of rays of light, it is impossible toobtain the effect of evenness of the diffraction efficiency includingthe shading of a ray of light.

[0011] On the other hand, in a photographing optical system to whichan-optical system of the invention is assumed to be applicable, as isunderstandable from FIGS. 1, 2 and 3 which are used for the descriptionof embodiments of the invention, an on-axial light flux and an off-axiallight flux are relatively unseparate from each other in the interior ofthe optical system. Accordingly, even on a lens surface distant from astop, the area of passing-through of the on-axial light flux and that ofthe off-axial light flux have a tendency to overlap each other.

[0012] Therefore, even if the structural arrangement disclosed in theabove Japanese Laid-Open Patent Application No. Hei 10-268115 is appliedto a photographic lens as it stands, it is difficult to obtain thediffraction efficiency excellent concurrently in respect of both theon-axial light flux and the off-axial light flux.

[0013] In particular, in the case of a photographic lens having arelatively-large aperture, since the respective areas, on which anon-axial light flux and an off-axial light flux are made incident, of alens surface having a diffraction grating provided thereon overlap eachother greatly, the above-mentioned difficulty increases.

BRIEF SUMMARY OF THE INVENTION

[0014] It is an object of the invention to provide an optical systemhaving such high optical performance that, when effecting achromatism bycombining a diffractive optical element and a refractive opticalelement, the diffraction efficiency excellent over the entire imageplane can be obtained even if light fluxes which are to reach respectivepositions of the image plane overlap each other greatly on a diffractiveoptical surface.

[0015] To attain the above object, in accordance with an aspect of theinvention, there is provided an optical system, comprising a diffractiveoptical element having a diffraction grating provided, on a lens surfacehaving curvature, in a concentric-circles shape rotationally-symmetricalwith respect to an optical axis, wherein a sign of the curvature of thelens surface having the diffraction grating provided thereon is the sameas a sign of a focal length, in a design wavelength, of a systemcomposed of, in the optical system, a surface disposed nearest to anobject side to a surface disposed immediately before the lens surfacehaving the diffraction grating provided thereon, and is different from asign of a distance from the optical axis to a position where a centerray of an off-axial light flux enters the lens surface having thediffraction grating provided thereon.

[0016] Here, the sign of the curvature of the lens surface is consideredpositive if the center of curvature exists on a light-exit side (imageside) with respect to the lens surface, and is considered negative ifthe center of curvature exists on a light-entrance side. (object side)with respect to the lens surface. Accordingly, the curvature of a lenssurface convex facing the object side (concave facing the image side)has the positive sign, and the curvature of a lens surface concavefacing the object side (convex facing the image side) has the negativesign. On the other hand, the sign of the distance from the optical axisto the position where a center ray of an off-axial light flux enters thelens surface having the diffraction grating provided thereon isconsidered positive if the position where the center ray enters the lenssurface exists on a side opposite to a side from which the center rayenters the optical system with respect to the optical axis, and isconsidered negative if the position where the center ray enters the lenssurface exists on the same side as a side from which the center rayenters the optical system with respect to the-optical axis. Accordingly,the distance from the optical axis to the position where a center ray ofan off-axial light flux enters the lens surface has the negative sign ifthe center ray enters the lens surface before crossing the optical axis,and has the positive sign if the center ray enters the lens surfaceafter crossing the optical axis.

[0017] Further, in accordance with another aspect of the invention,there is provided an optical system, comprising a diffractive opticalelement having a diffraction grating provided, on a lens surface havingcurvature, in a concentric-circles shape rotationally-symmetrical withrespect to an optical axis, wherein an apex of an imaginary cone formedby extending a non-effective surface of the diffraction grating (a sidesurface of the diffraction grating) is located adjacent to the center ofcurvature of the lens surface having the diffraction grating providedthereon.

[0018] These and further objects and features of the invention willbecome apparent from the following detailed description of preferredembodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0019]FIG. 1 is a sectional view of an optical system according to afirst embodiment of the invention.

[0020]FIG. 2 is a sectional view of an optical system according to asecond embodiment of the invention.

[0021]FIG. 3 is a sectional view of an optical system according to athird embodiment of the invention.

[0022]FIG. 4 is a sectional showing, in outline, a diffractive opticalsurface in which both a diffraction grating and a lens surface havingthe diffraction grating provided thereon are positive in optical power.

[0023]FIG. 5 is a schematic diagram showing, in outline, a diffractiveoptical surface in which both a diffraction grating and a lens surfacehaving the diffraction grating provided thereon are negative in opticalpower.

[0024]FIG. 6 is a sectional view showing, in outline, a single-layerdiffraction grating.

[0025]FIG. 7 is a graph showing the wavelength-dependent characteristicof the diffraction efficiency of the diffraction grating shown in FIG.6.

[0026]FIG. 8 is a sectional view showing, in outline, a laminateddiffraction grating (of the close-contact laminated type).

[0027]FIG. 9 is a graph showing the wavelength-dependent characteristicof the diffraction efficiency of the diffraction grating shown in FIG.8.

[0028]FIG. 10 is a sectional view showing, in outline, another laminateddiffraction grating (of the adjacently-laminated type with an airlayer).

[0029]FIG. 11 is a-table showing angles of incidence (θ), on adiffraction grating, of a center-ray and marginal rays (upper and lowerrays) among meridional rays of an on-axial light flux and an off-axiallight flux entering the optical system according to the first embodimentof the invention.

[0030]FIG. 12 shows graphs for the grating-pitch-dependentcharacteristics of the diffraction efficiency, in wavelengths 450 nm,550 nm and 650 nm, for a first-order diffracted light including shadingat a non-effective surface with respect to incident rays of light havingthe angles of incidence θ=0°, θ=−10° and θ=+10° in the diffractiongrating of the adjacently-laminated type of the optical system accordingto the first embodiment.

[0031]FIG. 13 is a schematic diagram showing, in outline, thearrangement of an optical apparatus (a single-lens-reflex camera)according to a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Hereinafter, preferred embodiments of the invention will bedescribed in detail with reference to the drawings.

[0033]FIGS. 1, 2 and 3 are sectional views showing the essential partsof optical systems OL according to first, second and third embodimentsof the invention, respectively. In the examples shown in FIGS. 1, 2 and3, the invention is applied to the photographing optical systems OL ofthe telephoto type, of the Gauss type and of the inverted telephototype, respectively. In FIGS. 1, 2 and 3, reference character DO denotesa diffractive optical surface which is a lens surface having adiffraction grating provided thereon. The diffraction grating is formedin a concentric-circles shape rotationally-symmetrical with respect toan optical axis, and the form of a section of the diffraction grating isthe blazed type. Reference character SP denotes-an aperture stop fordetermining the brightness of the optical system OL, and referencecharacter IP denotes an image plane.

[0034]FIGS. 4 and 5 are diagrams for explaining the diffractive opticalsurface according to the invention. In the state shown in each of FIGS.4 and 5, a diffraction grating 2 of the blazed type is provided on alens surface la on one side of a lens 1. In FIG. 4, reference numeral 4denotes an effective surface having a desired diffracting function ofthe diffraction grating, and reference numeral 3 denotes a side surfacewhich is obtained when forming the diffraction grating 2 of the blazedtype on the lens surface 1 a and does not take part in the diffractingfunction (hereinafter referred to as the “non-effective surface”).

[0035] The non-effective surface 3, when being imaginarily extended,intersects an optical axis 5 at one point, and is a part of the conicalsurface of an imaginary cone thus formed with such an intersection pointtaken as an-apex thereof. In FIG. 4, the apex of such an imaginary coneis denoted by reference character DOP, and the center of curvature ofthe lens surface la is denoted by reference character RC.

[0036] In each of the-first to third embodiments, the apex DOP is madeto be located adjacent to the center of curvature RC. Here, the locationadjacent to the center of curvature RC means satisfying the followingcondition:

|DL/R|<0.3

[0037] where DL is a distance from the apex DOP of the imaginary cone tothe center of curvature of the lens surface 1 a, which is a base havingthe diffraction grating provided thereon, and R is a radius of curvatureof the lens surface 1 a.

[0038] In each of the first to third embodiments, the directions ofincidence of rays from an on-axial object point (an on-axial light flux)on the lens surface, which is a base having the diffraction gratingprovided thereon, are respectively made approximately equal to thedirections of normal lines at intersection points where the raysintersect the lens surface having the diffraction grating providedthereon. For example, the height of each zone of the diffraction gratingis set to such a height that the diffraction efficiency of a ray oflight coincident with the optical axis becomes 100% (such a height thatan optical path length becomes integer times the design wavelength), andis made uniform in the directions of the normal lines of the lenssurface having the diffraction grating provided thereon. Thisarrangement makes it possible to set the diffraction efficiency for theon-axial light flux to almost 100%.

[0039] As a result, shading of the whole on-axial light flux isprevented, so that it is possible to heighten the diffractionefficiency.

[0040] First, the diffraction efficiency for the on-axial light fluxamong light fluxes to be diffracted by the diffractive optical surfacewill be described.

[0041] In each of the first to third embodiments, the sign of thecurvature Ra (i.e, 1/R) of the lens surface having the diffractiongrating provided thereon (for example, the sign of the curvature of aconcave surface facing the image side (the light-exit side) beingpositive and the sign of the curvature of a convex surface facing theimage side being negative) is made to be the same as the sign of a focallength “fa”, in the design wavelength, of a composite system composed ofa surface disposed nearest to the object side (the light-entrance side)to a surface disposed immediately before the lens surface having thediffraction grating provided thereon in the optical system. In the caseof the first embodiment shown in FIG. 1, both the curvature Ra and thefocal length “fa” have the positive sign, and in the cases of the secondand third embodiments shown in FIGS. 2 and 3, both the curvature Ra andthe focal length “fa” have the negative sign.

[0042] In this instance, the directions of incidence in which rays oflight constituting the on-axial light flux reaching the center of theimage plane enter the lens surface having the diffraction gratingprovided thereon respectively become approximately equal to thedirections of normal lines at intersection points where-the rays oflight intersect the lens surface having the diffraction grating providedthereon. In such a state, the apex DOP of the imaginary cone a part ofwhich is formed by the non-effective surface of the diffraction gratingis made to be located adjacent to the center of curvature RC (theposition on which the normal lines concentrate) of the lens surfacehaving the diffraction grating provided thereon. By this arrangement,shading of on-axial rays of light at the non-effective surface of thediffraction grating is lessened, so that the diffraction efficiency isprevented from lowering due to shading.

[0043] Next, the diffraction efficiency for the off-axial light fluxamong light fluxes to be diffracted by the diffractive optical surfacewill be described.

[0044] In each of the first to third embodiments, the sign of a distanceHa from the optical axis to a position where a center ray of theoff-axial light flux enters the lens surface having the diffractiongrating provided thereon (the sign of the distance Ha is consideredpositive if the position of incidence on the lens surface is on a sideopposite to the side from which the center ray enters the optical systemwith respect to the optical axis, and is considered negative if theposition of incidence on the lens surface is on the same side as theside from which the center ray enters the optical system with respect tothe optical axis, and, accordingly, the distance Ha has the positivesign if the center ray enters the lens surface after crossing theoptical axis.) is made to be different from the sign of the curvature Raof the lens surface having the diffraction grating provided thereon. Inthe case of the first embodiment, Ha<0 and Ra>0, and, in the cases ofthe second and third embodiments, Ha>0 and Ra<0.

[0045] In this instance, similarly to the case of the on-axial lightflux, the directions of incidence in which rays of light constitutingthe off-axial light flux reaching the margin of the image plane enterthe lens surface having the diffraction grating provided thereonrespectively become approximately equal to the directions of normallines at intersection points where the rays of light intersect the lenssurface having the diffraction grating provided thereon.

[0046] Accordingly, similarly to the case of the on-axial light flux,the apex DOP of the imaginary cone a part of which is for med by thenon-effective surface 3 of the diffraction grating is made to be locatedadjacent to the-center of curvature RC (the position on which the normallines concentrate) of the lens surface having the diffraction gratingprovided thereon. By this arrangement, shading of rays of light of theoff-axial light flux reaching the margin of the image plane at the.non-effective surface of the diffraction grating is lessened, so thatthe diffraction efficiency can be prevented from lowering.

[0047] Further, the directions of incidence in which rays of lightconstituting the off-axial light flux reaching the margin of the imageplane enter the lens surface having the diffraction grating providedthereon respectively become approximately equal to the directions ofnormal lines at intersection points where the rays of light intersectthe lens surface having the diffraction grating provided thereon.Therefore, it is possible to obtain the high diffraction efficiency evenwhile keeping the height of the zone of the diffraction grating decidedon the basis of the on-axial light flux as mentioned in the foregoing.

[0048] According to the invention, with the arrangement as describedabove, it is possible to lessen shading over the entire image plane,thus obtaining the high diffraction efficiency, and, in particular, tomitigate the adverse effect of color flare occurring due to unnecessarydiffracted rays of color light fluxes at the time of photographing ahigh-luminance object.

[0049] While, in the invention, with the various elements thereofdefined as described above, there is attained an optical system havinggood optical performance over the entire image plane, it is preferableto further satisfy at least one of the following structural conditions.

[0050] With-such a condition satisfied, it is possible to further lessenshading, at the non-effective surface of the diffraction grating, of alight flux reaching the center or thereabout of the image plane, whichis of relatively great importance in image quality, and it is possibleto further improve the diffraction efficiency for a light flux reachingthe center or thereabout of the image plane.

[0051] (A-1) The following condition is satisfied:

|D/R|·<5   (1)

[0052] where D is a distance from the position of the center ofcurvature of the lens surface having the diffraction grating providedthereon to the position of a focus, in the design wavelength, of acombined system composed of a surface disposed nearest to the objectside to a surface disposed immediately before the lens surface havingthe diffraction grating provided thereon in the optical system, and R isthe radius of curvature of the lens surface having the diffractiongrating provided thereon.

[0053] If the upper limit of the condition (1) is exceeded, thediffraction efficiency, in particular, around the center of the imageplane deteriorates disadvantageously. In the invention, it is morepreferable to alter the numerical range of the condition (1) as follows:

|D/R|<3   (1)′

[0054] thereby further improving the diffraction efficiency.

[0055] (A-2) The following condition is satisfied:

C ₁·P <0   (2)

[0056] where P is a refractive power of the lens surface having thediffraction grating provided thereon, and C₁ is a phase coefficient fora second-degree term when a phase shape of the diffraction grating isexpressed by the following equation:

φ(Y)=(2π/λ_(O))(C ₁ Y ² +C ₂ Y ⁴ +C ₃ Y ⁶+. . . )   (a)

[0057] where Y is the height in the vertical direction-from the opticalaxis, λ_(O) is the design wavelength, and C_(i) is a phase coefficient(i 1, 2, 3 . . . ).

[0058] The condition (2) is provided for manufacturing the diffractiongrating according to the invention with high accuracy.

[0059] The technical significance of the condition (2) will be describedbelow.

[0060] An optical power φ_(D)(λ, m) of the diffractive surface(corresponding to a refractive power and expressed by the reciprocal ofthe focal length) for an arbitrary wavelength λ and an arbitrarydiffraction order can be expressed by the following equation, using thephase coefficient C, in the above equation (a):

φ_(D)(λ, m)=−2 C ₁ mλ/m _(O)λ_(O)   (b).

[0061] Thus, an optical power of the diffractive surface for the designwavelength λ_(O) and the design diffraction order m_(O) becomes asfollows:

φ_(D)(λ_(O) , m _(O))=−2 C₁   (c).

[0062] In other words, the condition (2) means that the optical powerφ_(D)(λ_(O), m_(O)) of the-diffraction grating for the design wavelengthand the design diffraction order is set to have the same sign as thesign of the refractive power of the lens surface having the diffractiongrating provided thereon.

[0063] Further, the significance which such an arrangement gives in theinvention will be described below on the basis of two kinds of blazedshapes, between which the sign of the phase coefficient C₁ varies, withreference to FIGS. 4 and 5.

[0064]FIGS. 4 and 5 are sectional views respectively showing, inoutline, diffraction gratings which are formed into the different blazedshapes, between which the sign of the phase coefficient C₁ varies, onthe basis of the condition (1). In each of the diffraction gratingsshown in FIGS. 4 and 5, the sign of the refractive power P of the lenssurface la having the diffraction grating 2 provided thereon is set insuch a way as to satisfy the condition (2). In the case of thediffraction grating shown in FIG. 4, C₁<0 (the optical power of thediffraction grating having a positive value), and P>0. In the case ofthe diffraction grating shown in FIG. 5, C₁>0 (the optical power of thediffraction grating having a negative value), and P<0. As mentioned inthe foregoing, in FIG. 4, the diffraction grating 2 is provided on thelens surface la of the lens 1, and the apex DOP of the imaginary coneformed by the non-effective surface 3 of the diffraction grating 2 islocated adjacent to the center of curvature RC of the lens surface 1 ahaving the diffraction grating 2 provided thereon.

[0065] As the methods for manufacturing diffraction gratings of such ablazed shape, there are a method of performing press molding with a moldor the like while fusing glass at a high temperature, a method ofperforming press molding of ultraviolet-curable plastic or the like witha mold on the surface of a glass substrate or the like and curing theplastic with radiation of an ultraviolet ray, a method of moldingplastic with a mold together with a lens, etc. Further, there are amethod of forming a diffraction grating by directly cutting glass, and amethod of forming a diffraction grating of the finely-stepped shape bywet-etching or dry-etching S_(i)O₂ or the like.

[0066] As is apparent also from FIGS. 4 and 5, since the non-effectivesurface 3 of the diffraction grating 2 is a part of the conical surface,if the sign of the refractive power P of the lens surface having thediffraction grating provided thereon or the sign of the optical power ofthe diffraction grating is made opposite to that adopted in thediffraction grating shown in FIG. 4 or 5, i.e., if C₁·P>0, there arisevarious problems in terms of workability. For example, in using a mold,it is difficult to work the mold, and, at the time of transfer on themold, it is difficult for fused glass or plastic to intrude in thedirection of the depth of the grating, thus causing the transferabilityto deteriorate. Further, at the time of the mold release, in the methodof performing molding while fusing glass, the method of molding plasticwith a mold together with a lens, etc., the mold can not be released inthe worst state, and, at the most, a tip portion of the grating isdamaged, thus causing the diffraction efficiency to deteriorate. In themethod of transferring ultraviolet-curable plastic to a glass substrateor the like, while, since the viscosity of the plastic is relativelylow, the mold may be somehow released, a tip portion or thereabout ofthe grating is deformed due to the stress occurring at the time of themold release, thus also causing the diffraction efficiency todeteriorate.

[0067] In short, in the case of C₁·P>0, in whatever forming-method amongthe forming methods using a mold, the transferability of the mold, thereleasability of the mold, etc., are deteriorated and it becomesimpossible to obtain the desired diffraction efficiency. Therefore, itis preferable to satisfy the above condition (2).

[0068] (A-3) The diffraction grating is a diffraction grating of thelaminated type (a laminated diffraction grating).

[0069] (A-4) The laminated diffraction grating is a diffraction gratingof the adjacently-laminated type (an adjacently-laminated diffractiongrating) in which at least one thin air layer is included and twodiffraction gratings are disposed adjacent to each other across the thinair layer.

[0070] (A-5) The adjacently-laminated diffraction grating is providedbetween two adjacent lens surfaces having substantially the samecurvature, and is composed of three layers, i.e., in order from theobject side, a first layer, a second layer and a third layer, and thesecond layer is the thin air layer.

[0071] (A-6) The first layer and the third layer of theadjacently-laminated diffraction grating are formed withultraviolet-curable plastic.

[0072] The above-mentioned structural conditions (A-3) to (A-6) areprovided for defining the grating structure for heightening thediffraction efficiency over the entirety of the useful wavelengthregion. Each of the structural conditions (A-3) to (A-6) will bedescribed in detail below.

[0073] As the method of heightening the absolute value of thediffraction efficiency over the entirety of the useful wavelengthregion, there is known a diffraction grating of the laminated type inwhich a plurality of blazed-type diffraction gratings are disposed inclose contact with each other or adjacent to each other and therefractive indices and Abbe numbers of the materials of the respectivediffraction gratings, the depths of the gratings, etc., areappropriately set.

[0074]FIG. 7 is a graph showing the wavelength-dependent characteristicof the diffraction efficiency, mainly for the first-order diffractedlight, obtained when a light flux is made to vertically enter thesingle-layer blazed-type diffraction grating shown in FIG. 6. In theactual structure of the diffraction-grating, as shown in FIG. 6,ultraviolet-curable plastic is coated on the surface of a-base material11 to form a plastic portions and, on the plastic portion, there isformed a grating 12 having a grating thickness d arranged such that thediffraction efficiency for the first-order diffracted light at thewavelength of 530 nm becomes 100%. As is apparent from FIG. 7, thediffraction efficiency for the design order decreases accordingly as thewavelength goes away from the optimized wavelength of 530 nm, and,conversely, the diffracted light of the zero order and the second ordernear the design order increases. Such an increase of the diffractedlight of the order other than the design order causes flare, therebylowering the resolution of the optical system.

[0075] On the other hand, FIG. 9 is a graph showing thewavelength-dependent characteristic of the diffraction efficiency,mainly for the first-order diffracted light, obtained when a light fluxis made to vertically enter the diffraction grating of the close-contactlaminated type shown in FIG. 8. In the specific structure of thediffraction grating, as shown in FIG. 8, a first diffraction grating 13made from ultraviolet-curable plastic (nd=1.499, υd=54) is formed on abase material 11, and, on the first diffraction grating 13, there isformed a second diffraction grating 14 made from anotherultraviolet-curable plastic (nd=1.598, υd=28) is formed. In such acombination of those materials, the grating thickness d1 of the firstdiffraction grating 13 is set to 13.8 μm, and the grating thickness d2of the second diffraction grating 14 is set to 10.5 μm. As is apparentfrom FIG. 9, with the diffraction grating having the laminatedstructure, the diffraction efficiency for the design order becomes 95%or more over the entirety of the useful wavelength region.

[0076] As mentioned in the foregoing, with the diffraction grating ofthe laminated structure used as a diffraction grating according to eachof -the embodiments, it is possible to further improve opticalperformance. At the same time, with the above-described construction, itis possible to obtain the diffraction efficiency excellent inwavelength-dependent characteristic over the entire image plane.Therefore, it is preferable to use the diffraction grating of thelaminated structure.

[0077] Next, the influence of the non-effective surface of thediffraction grating on the diffraction efficiency will be describedbelow in respect of, among diffraction gratings of the laminatedstructure, a diffraction grating of the close-contact laminated type inwhich grating portions are in close contact with each other, and adiffraction grating of the adjacently-laminated type in which gratingportions are disposed adjacent to each other across a thin air layer.

[0078] In a case where the above two types of diffraction gratings aremade of the same material, since the depth of a grating portion requiredfor obtaining necessary diffracted light is inversely proportional tothe absolute value of a difference of refractive indices of surfaces onthe entrance side and the exit side of the grating portion, thediffraction grating-of the adjacently-laminated type, which has an airlayer, makes it possible to more reduce the total of depths of the wholegrating portion than the diffraction grating of the close-contactlaminated type.

[0079] As a result, the diffraction grating of the adjacently-laminatedtype more lessens the shading of rays on the non-effective surface thanthe diffraction grating of the close-contact laminated type, and is,therefore, advantageous in respect of an improvement on the diffractionefficiency.

[0080] Therefore, it is preferable to use the diffraction grating of theadjacently-laminated type, in which grating portions are disposedadjacent to each other across a thin air layer.

[0081] In this instance, if a lens which is a component of the opticalsystem is so divided into two parts that lens surfaces obtained by thisdivision have approximately the same curvature, and the diffractiongrating of the adjacently-laminated type is provided on each of the lenssurfaces, with the result that there are formed three layers, i.e., afirst layer, a second layer and a third layer in order from the objectside, the second layer being a thin air layer, it is possible to makethe diffraction efficiency more excellent with a relatively simpleconstruction without influencing the various aberrations of the entireoptical system.

[0082] In a case where one lens is so divided into two parts that lenssurfaces obtained by this division have approximately the samecurvature, the curvatures of the lens surfaces hardly contribute to thevarious aberrations even if being set to any values while being keptapproximately the same. Therefore, it is possible to set the curvaturesof the lens surfaces to values which are specialized for optimizing thediffraction efficiency around the center of the image plane and aroundthe margin of the image plane. Accordingly, it is preferable to use thediffraction grating of the adjacently-laminated type having such aconstruction.

[0083] In this instance, the glass materials of the divided lens partsneed not being the same, and may be varied according to necessity. Inaddition, if two lens surfaces on each of which the diffraction gratingis to be provided exist from the beginning, before the diffractiongrating is provided, for reasons of the structure for correctingaberration, such as chromatic aberration, spherical aberration or coma,and are such cemented surfaces that the conditions of curvature, etc.,satisfy the arrangement of the invention, it is unnecessary to newlydivide a lens, and the diffraction grating ought to be provided on theexisting lens surfaces.

[0084]FIG. 10 is an enlarged sectional view showing, in outline, a partof a diffraction grating of the adjacently-laminated type which isapplied to the optical system of the first embodiment. In FIG. 10,reference numerals 21 and 22 denote lens parts into which one lens is sodivided that lens surfaces obtained by this division have approximatelythe same curvature. Reference numerals 23 and 24 denote a diffractiongrating serving as a first layer (nd=1.6685, υd=19.7) and a diffractiongrating serving as a third layer (nd=1.5240, υd=50.8), respectively.Reference numeral 25 denotes an air layer serving as a second layer.Reference character O denotes an optical axis of the optical system.Further, reference characters P and Q denote rays incident on thediffraction grating 23. Reference character V denotes a normal line atan intersection point between the ray P (Q) and a lens surface 21 ahaving the diffraction grating 23 provided thereon. Reference charactere denotes an angle which the ray P (Q) makes with the normal line V ofthe lens surface 21 a, with a positive angle being taken clockwise inFIG. 10. Further, reference characters d1 and d2 denote the gratingthicknesses of the first layer and the 11 third layer, respectively,being set to d1=5 μm and d2=7.5 μm. Reference character Pi denotes thegrating pitch of the i-th zone of the first layer (the grating pitch ofthe i-th zone of the first layer being set equal to that of the thirdlayer). The minimum grating pitch, which is on the 155th zone at themost marginal portion of the diffraction grating, is 156 μm.

[0085] Incidentally, a diffraction grating proposed in Japanese PatentApplication No. Hei 11-213374 is applicable to the shape of thediffraction grating of the adjacently-laminated type.

[0086]FIG. 11 is a table showing angles of incidence (θ), on thediffraction grating, of a center ray and marginal rays (upper and lowerrays) among meridional rays of an on-axial light flux and an off-axiallight flux entering the optical system according to the firstembodiment. Further, FIG. 12 shows graphs for thegrating-pitch-dependent characteristics of the diffraction efficiency,in wavelengths 450 nm, 550 nm and 650 nm, for a first-order diffractedlight including shading at the non-effective surface of the diffractiongrating with respect to incident rays of light having the angles ofincidence θ=0°, θ=−10° and θ=+10° in the diffraction grating of theadjacently-laminated type shown in FIG. 10.

[0087] In general, the smaller the grating pitch, the more greatly aconverted optical path length of a ray passing through the gratingportion varies with respect to a change of the angle of incidence of theray, and, therefore, the more remarkable the deterioration of thediffraction efficiency becomes. As shown in FIG. 12, the diffractionefficiency of the diffraction grating of the adjacently-laminated typewhich is applied to the optical system according to the first embodimentis excellent including the wavelength-dependent characteristic.

[0088] Then, as shown in FIG. 11, the angle of incidence θ of array inthe optical system of the first embodiment takes the largest absolutevalue for the marginal ray (lower ray) of the maximum angle of view (thehalf angle of view being 3.16°), i.e., θ=+2.50°. Further, the positionwhere the marginal ray passes through is the most marginal part of thegrating portion, and, as mentioned in the foregoing, the grating pitchof the grating portion becomes a minimum value Pi=156 μm (i=155) at sucha position. While the diffraction efficiency for a ray passing throughthat position becomes lowest, it is understood from FIG. 12 that thehigh efficiency equivalent to the diffraction efficiency for an incidentray having the angle of incidence θ=0° can be kept.

[0089] Next, the method for forming the laminated-type diffractiongrating and the degree of freedom of design for the aberrationcorrection and the diffraction efficiency will be described.

[0090] One of purposes of introducing the diffraction grating to anoptical system having a refractive member, such as a lens, is to cancel,with the diffraction grating, chromatic aberration occurring at therefractive optical system.

[0091] Accordingly, in a case where the diffraction grating is to beprovided on the surface of a lens, since the lens takes partial chargeof chromatic aberration required as a refractive optical system withrespect to the partial charge of chromatic aberration by the diffractiongrating, it is difficult to arbitrarily select the material of the lens.

[0092] More specifically, in the case of the method of press-molding,with a mold or the like, the diffraction grating together with a lenswhile fusing glass at a high temperature, or in the case of the methodof molding,. with a mold, the diffraction grating together with a lensusing plastic, the lens and the diffraction grating become made of thesame material, so that the degree of freedom of selecting the materialof the diffraction grating is lost. Therefore, it becomes difficult tomake the correction of chromatic aberration compatible with theimprovement of the wavelength-dependent characteristic of thediffraction efficiency in the laminated-type diffraction grating.

[0093] Accordingly, if such a method as to enable the materials of thelens and the diffraction grating to be selected independent of eachother, for example, the method of forming the diffraction grating withultraviolet-curable plastic, is used, it becomes possible to obtain anoptical system excellent both in the correction of chromatic aberrationand the diffraction efficiency. Therefore, such a forming method oughtto be used.

[0094] In addition, the optical system according to the invention isapplicable widely to an image pickup apparatus, such as a film camera, avideo camera, a digital camera or the like, an observation apparatus,such as a telescope, a binocular or the like, a stepper (a projectionexposure apparatus) for manufacturing semiconductor devices, a varietyof optical measuring apparatuses, etc.

[0095] Here, an embodiment in which the optical system according to eachof the first to third embodiments is applied to an optical apparatuswill be described with reference to FIG. 13.

[0096]FIG. 13 is a schematic diagram showing, in outline, the essentialparts of a single-lens reflex camera. In FIG. 13, reference numeral 10denotes a photographic lens having an optical system 1 according to anyone of the first to third embodiments. The optical system 1 is held by alens barrel 2 serving as a holding member. Reference numeral 20 denotesa camera body, which is composed of a quick-return mirror 3 arranged toreflect upward a light flux coming from the photographic lens 10, afocusing screen 4 disposed on the image forming position of thephotographic lens 10, a pentagonal roof prism 5 arranged to convert aninverted image formed on the focusing screen 4 into an erecting image,an eyepiece lens 6 provided for observing the erecting image, etc.Reference numeral 7 denotes a film surface. In taking a picture, thequick-return mirror 3 retreats from the optical path and a shutter (notshown) is opened, so that an image is formed on the film surface 7 bythe photographic lens 10.

[0097] The advantageous effects mentioned in each of the first to thirdembodiments are effectively enjoyed by such an optical apparatus asdisclosed in the present embodiment.

[0098] Next, numerical data of the numerical examples 1 to 3 of opticalsystems corresponding to the first to third embodiments of the inventionare shown.

[0099] In the numerical data of the numerical examples 1 to 3, f denotesthe focal length, Fno denotes the F-number, ω denotes a half angle ofview, ri denotes the radius of curvature of the i-th surface, whencounted from the object side, di denotes the separation between the i-thsurface and the (i+1)th surface, when counted from the object side, niand υi respectively denote the refractive index and Abbe number of thei-th optical member, when counted from the object side.

[0100] The shape of an aspheric surface is expressed in the coordinateswith an X axis in the optical axis direction (the direction in whichlight advances) and an H axis in the direction perpendicular to theoptical axis, with the intersection point between the aspheric surfaceand the X axis taken as the~original point, by the following equation:$X = {\frac{H^{2}/r}{1 + \sqrt{1 - \left( {H/r} \right)^{2}}} + {A\quad H^{2}} + {B\quad H^{4}} + {C\quad H^{6}} + {D\quad H^{8}} + {E\quad H^{10}} + {F\quad H^{12}}}$

[0101] where r is the radius of curvature of a paraxial portion of theaspheric surface, and A, B, C, D, E and F are aspheric coefficients.

[0102] Further, the coefficients C₁, C₂ . . . of the shape of thediffraction grating are shown on the basis of the above-mentionedequation (a). In addition, the indication “D-X” means “×10^(−X”.)

[0103] In addition, the values of the factors in the above-mentionedconditions for the numerical examples 1 to 3 are listed in Table-1.Numerical Example 1: f = 392.00  Fno = 1:4.12  2ω = 6.32° r 1 = 115.683d 1 = 9.40 n 1 = 1.56384 ν 1 = 60.7 r 2 = 319.640* d 2 = 9.00 n 2 =1.51633 ν 2 = 64.1 r 3 = −478.031 d 3 = 16.76 r 4 = 96.413 d 4 = 8.60 n3 = 1.51823 ν 3 = 58.9 r 5 = 472.518 d 5 = 3.11 r 6 = −495.228 d 6 =3.60 n 4 = 1.74950 ν 4 = 35.3 r 7 = 135.791 d 7 = 4.08 r 8 = 71.132 d 8= 8.40 n 5 = 1.48749 ν 5 = 70.2 r 9 = 245.218 d 9 = 0.80 r10 = 51.628d10 = 5.30 n 6 = 1.67270 ν 6 = 32.1 r11 = 40.134 d11 = 39.15 r12 =1141.040 d12 = 1.80 n 7 = 1.43387 ν 7 = 95.1 r13 = 56.180 d13 = 22.84r14 = ∞ (Stop) d14 = 10.50 r15 = 90.269 d15 = 1.30 n 8 = 1.80518 ν 8 =25.4 r16 = 34.628 d16 = 4.70 n 9 = 1.48749 ν 9 = 70.2 r17 = −87.040 d17= 0.50 r18 = 68.906 d18 = 3.85 n10 = 1.76182 ν10 = 26.5 r19 = −59.203d19 = 1.30 n11 = 1.80400 ν11 = 46.6 r20 = 32.357 d20 = 3.41 r21 =−74.136 d21 = 1.30 n12 = 1.80400 ν12 = 46.6 r22 = 131.844 d22 = 1.53 r23= 75.659 d23 = 6.20 n13 = 1.63980 ν13 = 34.5 r24 = −31.349 d24 = 1.40n14 = 1.80400 ν14 = 46.6 r25 = −170.115 d25 = 14.65 r26 = 82.731 d26 =6.60 n15 = 1.51633 ν15 = 64.1 r27 = −106.118 d27 = 0.72 r28 = ∞ d28 =2.20 n16 = 1.51633 ν16 = 64.1 r29 = ∞

[0104] Numerical Example 2: f = 51.50  Fno = 1:1.46  2ω = 45.57° r 1 =50.061 d 1 = 4.15 n 1 = 1.78000 ν 1 = 50.0 r 2 = 243.413 d 2 = 0.10 r 3= 30.828 d 3 = 3.68 n 2 = 1.88500 ν 2 = 41.0 r 4 = 45.316 d 4 = 1.98 r 5= 57.891 d 5 = 3.40 n 3 = 1.65070 ν 3 = 31.8 r 6 = 20.182 d 6 = 5.87 r 7= ∞ (Stop) d 7 = 10.63 r 8 = −19.290 d 8 = 3.40 n 4 = 1.79528 ν 4 = 28.1r 9 = 131.601 d 9 = 6.77 n 5 = 1.79558 ν 5 = 48.3 r10 = −32.350* d10 =1.10 r11 = −106.926 d11 = 5.26 n 6 = 1.88430 ν 6 = 40.4 r12 = −36.060d12 = 0.10 r13 = 82.121 d13 = 3.60 n 7 = 1.88500 ν 7 = 41.0 r14 =455.723

[0105] Numerical Example 3: f = 24.61  Fno = 1:1.45  2ω = 82.64° r 1 =62.320 d 1 = 2.80 n 1 = 1.69680 ν 1 = 55.5 r 2 = 31.224 d 2 = 5.77 r 3 =58.654 d 3 = 2.30 n 2 = 1.69680 ν 2 = 55.5 r 4 = 32.021 d 4 = 6.94 r 5 =220.248 d 5 = 4.36 n 3 = 1.71300 ν 3 = 53.8 r 6 = −101.757 d 6 = 4.09 r7 = 72.682 d 7 = 2.96 n 4 = 1.84666 ν 4 = 23.8 r 8 = 340.356 d 8 = 1.70n 5 = 1.49700 ν 5 = 81.6 r 9 = 23.248 d 9 = 12.21 r10 = 30.622 d10 =6.82 n 6 = 1.80400 ν 6 = 46.6 r11 = 58.996 d11 = 0.15 r12 = −1174.273d12 = 1.48 n 7 = 1.72825 ν 7 = 28.5 r13 = 38.385 d13 = 4.63 r14 = ∞(Stop) d14 = 7.99 r15 = 16.394 d15 = 4.66 n 8 = 1.84666 ν 8 = 23.9 r16 =−37.366** d16 = 0.15 r17 = −201.177 d17 = 7.04 n 9 = 1.60311 ν 9 = 60.7r18 = −23.231* d18 = 0.15 r19 = −86.014 d19 = 5.55 n10 = 1.77250 ν10 =49.6 r20 = −29.191

[0106] A =  0.00000D+00 B =  2.10265D−05 C =  1.79508D−08 D =−1.59961D−11 E = −1.82408D-13 F =  2.10282D−16 C₁ = −4.42491D−04 C₂ = 3.53506D−07 C₃ =  3.46931D−11 C₄ =  2.27434D−12 C₅ = −2.17625D−15 C₆ = 9.09469D−18 C₇ = −2.48396D−20 C₈ =  9.80318D−23 C₉ = −1.98660D−24 C₁₀ = 8.60378D−27

[0107] TABLE 1 Condition Numerical Example No. Factor 1 2 3 (1) |D/R|0.026 1.172 2.801 (2) C₁ · P 6.29847D−09 −6.20948D−06 −1.14875D−05

[0108] With the above-described elements defined as set forth in each ofthe embodiments, it is possible to attain an optical system having suchhigh optical performance that, when effecting achromatism by combining adiffractive optical element and a refractive optical element, thediffraction efficiency excellent over the entire image plane can beobtained even if light fluxes which are to reach respective positions ofthe image plane overlap each other greatly on a diffractive opticalsurface.

1. An optical system, comprising: a diffractive optical element having adiffraction grating provided, on a lens surface having curvature, in aconcentric-circles shape rotationally-symmetrical with respect to anoptical axis, wherein a sign of the curvature of the lens surface havingsaid diffraction grating provided thereon is the same as a sign of afocal length, in a design wavelength, of a system composed of, in saidoptical system, a surface disposed nearest to an object side to asurface disposed immediately before the lens surface having saiddiffraction grating provided thereon, and is different from a sign of adistance from the optical axis to a position where a center ray of anoff-axial light flux enters the lens surface having said diffractiongrating provided thereon.
 2. An optical system according to claim 1,wherein an apex of an imaginary cone formed by extending a non-effectivesurface of said diffraction grating is located adjacent to the center ofcurvature of the lens surface having said diffraction grating providedthereon.
 3. An optical system according to claim 2, wherein said opticalsystem satisfies the following condition: |DL/R|<0.3 where DL is adistance from the apex of the imaginary cone to the center of curvatureof the lens surface having said diffraction grating provided thereon,and R is a radius of curvature of the lens surface having saiddiffraction grating provided thereon.
 4. An optical system according toclaim 1, wherein said optical system satisfies the following condition:|D /R|<5 where D is a distance from the center of curvature of the lenssurface having said diffraction grating provided thereon to a focus, inthe design wavelength,.of the system composed of, in said opticalsystem, the surface disposed nearest to the object side to the surfacedisposed immediately before the lens surface having said diffractiongrating provided thereon, and R is a radius of curvature of the lenssurface having said diffraction grating provided thereon.
 5. An opticalsystem according to claim 1, wherein said optical system satisfies thefollowing condition: C₁·P<0 where P is a refractive power of the lenssurface having said diffraction grating provided thereon, and C₁ is aphase coefficient for a second-degree term when a phase shape of saiddiffraction grating is expressed by the following equation:φ(Y)=(2π/λ_(O))(C ₁ Y ² +C ₂ Y ⁴ +C ₃ Y ⁶+. . . ) where Y is the heightin a vertical direction from the optical axis, λ_(O) is the designwavelength, and C_(i) is a phase coefficient (i=1, 2, 3 . . . ).
 6. Anoptical system according to claim 1, wherein said diffraction grating isa laminated diffraction grating.
 7. An optical system according to claim6, wherein said laminated diffraction grating is an adjacently-laminateddiffraction grating in which two diffraction gratings are disposedadjacent to each other across an air layer.
 8. An optical systemaccording to claim 7, wherein said adjacently-laminated diffractiongrating is provided between two adjacent lens surfaces havingsubstantially the same curvature, and is composed of three layers,including, in order from the object side, a first layer, a second layerand a third layer, said second layer being the thin air layer.
 9. Anoptical system according to claim 7, wherein each of the two diffractiongratings of said adjacently-laminated diffraction grating are formedwith ultraviolet-curable plastic.
 10. An optical system according toclaim 1, wherein said optical system comprises a plurality ofdiffractive optical elements each-of which is equivalent to saiddiffractive optical element.
 11. An optical system according to claim 1,wherein said diffraction grating is a blazed-type diffraction grating.12. An optical system, comprising: a diffractive optical element havinga diffraction grating provided, on a lens surface having curvature, in aconcentric-circles shape rotationally-symmetrical with respect to anoptical axis, wherein an apex of an imaginary cone formed by extending anon-effective surface of said diffraction grating is located adjacent tothe center of curvature of the lens surface having said diffractiongrating provided thereon.
 13. An optical system according to claim 12,wherein said optical system satisfies the following condition:|DL/R|<0.3 where DL is a distance from the apex of the imaginary cone tothe center of curvature of the lens surface having said diffractiongrating provided thereon, and R X is a radius of curvature of the lenssurface having said diffraction grating provided thereon.
 14. An opticalsystem according to claim 12, wherein said optical system satisfies thefollowing condition: |D/R|<5 where D is a distance from the center ofcurvature of the lens surface having said diffraction grating providedthereon to a focus, in a design wavelength, of a system composed of, insaid optical system, a surface disposed nearest to an object side to asurface disposed immediately before the lens surface having saiddiffraction grating provided thereon, and R is a radius of curvature ofthe lens surface having said diffraction grating provided thereon. 15.An optical system according to claim 12, wherein said optical systemsatisfies the following condition: C ₁ ·P<0 where P is a refractivepower of the lens surface having said diffraction grating providedthereon, and C₁ is a phase coefficient for a second-degree term when aphase shape of said diffraction grating is expressed by the followingequation: φ(Y)=(2π/λ_(O))(C ₁ Y ² +C ₂ Y ⁴ +C ₃ Y ⁶+. . . ) where Y isthe height in a vertical direction from the optical axis, λ_(O) is adesign wavelength, and C_(i) is a phase coefficient (i=1, 2, 3 . . . ).16. An optical system according to claim 12, wherein said diffractiongrating is-a laminated diffraction grating.
 17. An optical systemaccording to claim 16, wherein said laminated diffraction grating is anadjacently-laminated diffraction grating in which two diffractiongratings are disposed adjacent to each other across an air layer.
 18. Anoptical system according to claim 17, wherein said adjacently-laminateddiffraction grating is provided between two adjacent lens surfaceshaving substantially the same curvature, and is composed of threelayers, including, in order from an object side, a first layer, a secondlayer and a third layer, said second layer being the thin air layer. 19.An optical system according to claim 17, wherein each of the twodiffraction gratings of said adjacently-laminated diffraction gratingare formed with ultraviolet-curable plastic.
 20. An optical systemaccording to claim 12, wherein said optical system comprises a pluralityof diffractive optical elements each of which is equivalent to saiddiffractive optical element.
 21. An optical system according to claim12, wherein said diffraction grating is a blazed-type diffractiongrating.
 22. An optical apparatus, comprising: an optical systemaccording to claim 1 or 12.